1. Field of the Invention
The present invention relates to an electrophotographic toner. More particularly, the invention relates to an electrophotographic toner that is able to produce a high quality image with uniform density without the contamination of image due to a non-control toner. The electrophotographic toner of the invention does not generate filming action on a photoconductor that is normally due to contamination by an external additive. The toner also maintains charge quantity and charge distribution stably over a long period of time in an electrophotographic image forming apparatus.
2. Description of the Related Art
Currently, electrophotographic image processing machines such as a laser printer, a facsimile and a copying machine, etc. are widely used. The image forming apparatus forms a latent image on a photoconductor by using a laser, migrate a toner onto the latent image formed on the photoconductor by employing electric potential difference, and transfer the latent image to a printing medium such as a paper to give a desired image.
FIG. 1 illustrates an example of a non-contact developing type electrophotographic image forming apparatus, of which operating mechanism is described below.
A non-magnetic one-component toner 8 is fed on a developing roller 5 by a feeding roller 6 made of an elastic member such as polyurethane foam or a sponge, and the like. The toner 8 supplied to the developing roller 5 arrives at the contacting part between a toner control blade 7 and the developing roller 5 through the rotation of the developing roller 5. The toner control blade 7 is made of an elastic member such as metal or a rubber, and the like. When the toner 8 is passed through the contacting area of the toner control blade 7 and the developing roller 5, the toner 8 is controlled and formed in a uniform layer to form a thin layer and to charge the toner sufficiently. Thin layered toner 8 migrates from the developing roller 5 to a developing area on the photoconductor 1 where the toner 8 is developed on an electrostatic latent image on the photoconductor 1.
The developing roller 5 is spaced from the photoconductor 1, without contacting each other in some intervals. The developing roller 5 rotates in a counterclockwise direction, and the photoconductor 1 rotates in a clockwise direction. The toner 8 that migrates to the developing area is developed on the electrostatic latent image of the photoconductor 1 by electric power generated by electric potential difference between the DC-offset AC voltage applied to the developing roller 5 and the electric potential of the latent image on the photoconductor 1.
The toner 8 developed on the photoconductor 1 arrives at a position of a transferring means 9 by the rotating direction of the photoconductor 1. The toner developed on the photoconductor 1 is transferred on a printing paper 13, which is passed between the photoconductor 1 and the transfer means by the transferring means 9 on which high voltage having counter polarity to the toner 8 is applied through corona discharge or in roller form, to form an image.
While the image transferred to the printing paper is passed through a fusing apparatus (not shown) at high temperature and high pressure, the toner is melted on the printing paper and thus the image is fused thereon. Meanwhile, the undeveloped toner remaining on the developing roller 5 is recovered by the feeding roller 6 in contact with the developing roller 5.
Such procedures are repeated to form a desired image.
Recently, high quality of images are required as electrophotographic image forming apparatuses such as an electrophotographic LBP, a multifunction machine and a color-copying machine are widely distributed. A toner used in a developing apparatus for this purpose is intended to possess the properties such as stable charge quantity and developing efficiency, and provide anti-fogging even during environmental changes and extended image printing times.
As an approach to control the stabilization of charge quantity in a toner, anti-fogging, and enhancing developing efficiency, and the like, various external additives such as silica, titanium oxide or titanium dioxide (TiO2), aluminum oxide (A2O3), strontium titanate (SrTiO3), barium titanate (BaTiO3) and calcium titanate (CaTiO3) are added to a toner, but there is a limitation in improving the image quality. That is, variation in charging property of a toner to environmental changes such as low temperature-low humidity and high temperature-high humidity occurs frequently, and uniform charge quantity and distribution is maintained in initial printing, but the drop of charge quantity occurs greatly during standing. Further, the drop of image density, fogging and toner scattering occur due to the drop of charge quantity and non-uniform charge distribution in image printing over long periods.
Accordingly, the types of various external additives to be added for improving image quality are increasing, and the amount of external additives added is progressively increasing. These external additives should maintain continuously stable adhesion state on a toner surface during long term printing, however, they are, in fact, impregnated into toner particles or some of them cause the contamination of a developing member due to their separation or departing, or the contamination of images therefrom. It has been found that such separation or departing of the external additives increases as the particle size of these external additives increases and the cohesive force between external additives increases. The separation becomes more severe due to the increase in the types and amounts of the external additives.
Particularly, a long lifetime of more than 10,000 copies is required for the recent color image forming apparatus. In these apparatus, the internal contamination in the image forming apparatus due to the external additives becomes more severe, thereby limiting the lifetime of the apparatus. Further, an image forming apparatus, in particular, the compaction in the inside member of a developing device and the roller type is promoted due to the need of compaction to set size, and in this case, the drop of image density and uniformity, and the contamination of a non-image part tends to occur easily due to the lack of absolute charge quantity and charging speed.